Enabling low-drift flexible perovskite photodetectors by electrical modulation for wearable health monitoring and weak light imaging

Metal halide perovskites are promising for next-generation flexible photodetectors owing to their low-temperature solution processability, mechanical flexibility, and excellent photoelectric properties. However, the defects and notorious ion migration in polycrystalline metal halide perovskites often lead to high and unstable dark current, thus deteriorating their detection limit and long-term operations. Here, we propose an electrical field modulation strategy to significantly reduce the dark current of metal halide perovskites-based flexible photodetector more than 1000 times (from ~5 nA to ~5 pA). Meanwhile, ion migration in metal halide perovskites is effectively suppressed, and the metal halide perovskites-based flexible photodetector shows a long-term continuous operational stability (~8000 s) with low signal drift (~4.2 × 10−4 pA per second) and ultralow dark current drift (~1.3 × 10−5 pA per second). Benefitting from the electrical modulation strategy, a high signal-to-noise ratio wearable photoplethysmography sensor and an active-matrix photodetector array for weak light imaging are successfully demonstrated. This work offers a universal strategy to improve the performance of metal halide perovskites for wearable flexible photodetector and image sensor applications.

The discussion of the contact resistance at the interface between the electrode and the MHP surface, and the injection barrier associated with the difference in work function is important, but not much is mentioned in this paper. What results would be obtained if other electrodes were used, such as those with different work functions or transparency?

3:
Baseline drift is clearly visible in many experimental results, but please provide a more detailed description of its origin. Also, baseline drift is often a transient characteristic, which means that the slope may change over time. This study does not show a relationship between this drift and time. I would like to see experimental results and more detailed discussion regarding the drift, such as whether it is a thermal effect or the effect of crystal defect trapping.

4:
The dark current in the proposed FPD can be suppressed by the electric field effect, but the crosstalk and weak leakage current in the wiring and FPD with three electrodes will occur when the active matrix structure shown in Figure 5 is used. The higher the resolution, the more leakage current associated with crosstalk at wirings and/or electrodes is expected to increase.
In the case of readout using such an active matrix structure, the total amount of dark current as a system is expected to be larger, mainly due to the crosstalk and leakage current of the wiring, not the dark current of each individual optical sensor. Please provide experimental results and discuss the dark current as a system. The experimental results (absolute value of dark current as a system) are not shown in Figure 5 and other experiments, but they are very important for applications.
Reviewer #2 (Remarks to the Author): The manuscript of Tang and co-workers reports flexible perovskite photodetectors with ultra-low dark current. The authors described a novel device architecture comprising an addition of a control electrode in a photoconductor configuration, which controls dark current. This mechanism is well justified by simulation and data in the manuscript and, therefore, very convincing. Finally, the authors proved the concept with flexible substrates for wearable bio-signal detection and in an imager architecture. These real-world applications make the manuscript very appealing for the perovskite community and, more in general, to research groups in the semiconductor field. I have some comments that should be considered before accepting the manuscript for publication in Nature Communications.
1) The advantage of this structure should be better described compared to for example a TFT configuration.
2) Hysteresis data must be reported, I am expecting that without selective contacts, the hysteresis is quite high for this configuration.
3) Why has the same contact (Au) been used for the control and ground and signal electrodes? What would happen in case of different metal contact? 4) A cross-section SEM will help the description of the configuration. 5) Similarly, SEM will be beneficial to appreciate the quality of the perovskite layers coated on top of the control electrode.
Reviewer #3 (Remarks to the Author): Photodetectors can directly convert optical signals into electrical signals, and play an important role in automatic driving, environmental monitoring, medical imaging, and military fields. Perovskite material has excellent photoelectric performance of direct band gap, large absorption coefficient and electron hole diffusion length , which opens a new door for the research of photodetectors. In this paper, The authors propose a significant electric field modulation strategy, based on which the dark current is significantly reduced and the ion migration is effectively suppressed. However, there are still problems in mechanism innovation and conclusion verification. The details are as follows: 1. In a recent paper by the author, a similar control method was used, that is, the introduction of additional control electrodes to suppress the dark current of the X-ray detector. Compared with this manuscript, what are the innovations in physical mechanism and technical methods? 2. The author introduces a third electrode on the other side of the photoconductor to suppress the dark current. The introduced third electrode is similar to the gate in the transistor, and What is the difference between both them? 3. The author mentioned that the iodine ion migration was inhibited by introducing the control electrode, but no direct evidence was given to prove this. 4. Is the response time related to the control electrode voltage? 5. What is the carrier mobility of the device in the thin film transistor array? Compared with similar devices, does the carrier mobility increase due to the introduction of the third electrode?
6. In Fig. 3h and Fig. 5f, the reason why the photocurrent rises first and then falls? 7. The author shows a response pulse of photoconductivity under extremely weak light intensity in the supporting information Fig. 10. It is suggested that the author increase at least 10 stable response periods to increase credibility. 8. Influence of film quality on voltage selection of control electrode needs to be pointed out.

Responses to NCOMMS-23-06143
We appreciate all reviewers' constructive comments and valuable suggestions. We have carefully read through the reviewers' comments and thoroughly addressed all the requests and concerns. And hopefully, our responses can release your concerns.
Following are our point-by-point responses:   On the other hand, the employment of control electrodes-defined and patterned by photolithography-did reduce the area of the "aperture". To obtain higher-resolution images, the footprints of these electrodes need further downscaling.

REVIEWER COMMENTS
Following the suggestion and question, we fabricated the FPDs with different perovskite channel lengths (100, 50, and 25 μm), and the electrodes were fixed at 1000 μm in length and 100 μm in width (with an area of 2×10 -3 cm 2 ). And the ratio of the electrode area ) to the light-receiving area ( ) is 2, 4, 8, respectively. As shown in Fig. R2a, the red dashed area represents the light receiving area ( ), and the blue dashed area represents the area of the whole device ( ).  Table R1.
5 Response: We thank the reviewer for providing this thoughtful comment. The work function of metal electrodes and the band structure of perovskite is very important for charge carrier injection and transport.
First, the bandgap of perovskite can be extracted to be 1.54 eV from the PL spectra by:

= 1240
Where is the peak wavelength (803 nm  In addition, we fabricated a device with asymmetric electrodes (Ag/perovskite/Au), whose current-voltage curve deviated from the origin ( In addition, we also fabricated devices with transparent ITO electrodes.    (2015)]. In order to better illustrate the relationship between baseline drift and time, we extracted the dark current variation with time in Fig. 3g,h, as shown in Fig.   R7a,b. By control voltage modulation, not only the value of the dark current is reduced, but also the ion migration near the signal electrode is suppressed, thus resulting a more stable baseline (Fig. R7b).  (2016)]. The relationship between the ion migration rate ( ) and temperature ( ) can be expressed by the following expression: Where is activation energy and is Boltzmann's constant.
Following the suggestion on the studying the effect of temperature on ion migration, we measured the current-time curves of FPD at different temperatures (from -30 to 50 °C). As shown in Fig. R7c, as the test temperature increases, the dark current increases and the baseline drift becomes more obvious. In addition, ion migration in polycrystalline films mostly occurs 9 via point defects or grain boundaries, due to the smaller ion activation energy at defect sites [Nat. Commun. 10, 1989(2019]. On the other hand, to study the effect of crystal defect trapping on ion migration, we fabricated devices based on MAPbBr3 single crystal perovskite. As shown in Fig. R7d, due to its lower defect density, the baseline current value of single crystal perovskite is very low (~0.3 nA, an order of magnitude lower than that of polycrystalline perovskite), and no significant drift was observed.
Therefore, both crystal defects and temperature could contribute to the ion migration and thus lead to baseline drift. Fig. R7 has been added as Supplementary Figure 16 to the revised version.  Therefore, we use a transistor in series with a perovskite photodetector to construct an active matrix FPD (AM-FPD) array, and the corresponding equivalent circuit diagram is shown in Fig. R9a. In such an array, each photodetector pixel is controlled and addressed by a TFT, providing arbitrary selecting, and avoiding signal crosstalk. During the testing process, the corresponding row and column electrodes are scanned at a certain frequency (similar to the refreshing rate of a cellphone display) and selected through matrix switch module (NI PXIe-2531) to turn on a specific transistor, thereby reading the perovskite photodetector signal. Take photodetector pixel R16,16 in the array as an example, we analyze the addressing process and the origin of dark current in the following. The gate voltage (VGS=20 V) and drain voltage (VDS=0.1 V) are respectively applied to the 16 th row (orange line) and 16 th column (blue line) 12 to read the signal of R16,16 (Fig. R9b). Since other transistors are in the off state (the gate voltage is not applied), the current in the circuit is almost all contributed by R16,16 at this time. The simplified equivalent circuit diagram of the addressed single pixel is shown in Fig. R9ci. The transistor is turned on at this time, so the transistor can be equivalent to a resistor (RTFT), whose resistance value is much smaller than the perovskite photoconductor (RTFT≪R16,16). As shown in Fig. R9cii and R8ciii, when the control voltage is applied, the dark current (blue arrow, IDark current) measured by the instrument is shunted by the control electrode, so the measured IDark current value can be significantly reduced.
In the actual test, the absolute value of the measured dark current can also be reduced to ~5 pA, but its random noise is significantly higher than the current signal measured directly on the perovskite, which is mainly due to the environmental noise and transistor device noise (Fig.   R9d). In future applications, the detection performance of the array can be improved by improving the quality of addressing transistors, optimizing the circuit wiring structure, and enhancing the electromagnetic shielding ability of the device to obtain higher-quality current data. The corresponding discussion and data have been supplemented in the revised manuscript (page 14, line 320) and supplementary material (Fig. S25).

Reviewer #2 (Remarks to the Author):
The manuscript of Tang and co-workers reports flexible perovskite photodetectors with ultralow dark current. The authors described a novel device architecture comprising an addition of a control electrode in a photoconductor configuration, which controls dark current. This mechanism is well justified by simulation and data in the manuscript and, therefore, very convincing. Finally, the authors proved the concept with flexible substrates for wearable biosignal detection and in an imager architecture. These real-world applications make the manuscript very appealing for the perovskite community and, more in general, to research groups in the semiconductor field. I have some comments that should be considered before accepting the manuscript for publication in Nature Communications.

Response:
We would like to gratefully thank the reviewer for your important comments and for your evaluation. The manuscript has been revised with full consideration of your comments.

Q1:
The advantage of this structure should be better described compared to for example a TFT configuration.
Response: Thanks for your kind suggestion. The typical configuration of a bottom gate top contact (BGTC) transistor is shown in Fig. R10. Compared with transistors configuration, our proposed structure has no dielectric layer and the control electrode is in direct contact with the semiconductor material. The control electrode in our structure can directly participate in the current transport process between the source and drain electrodes, so it has a stronger control effect on the dark current, and the value of the dark current can be reduced to zero or even a negative value (Fig. 2c). Therefore, our structure can achieve lower dark current values compared to a TFT configuration. Corresponding comments have been added to the revised manuscript (page 3, line 70) and supplementary material (Fig. S1).   (2022)]. The corresponding discussion and data have been added to the revised manuscript (page 12, line 268) and supplementary material (Fig. S24).

Q3: Why has the same contact (Au) been used for the control and ground and signal electrodes?
What would happen in case of different metal contact?
Response: We thank the reviewer for this comment. The reason we use the Au electrode is that it has better chemical stability than other electrodes, and its work function matches that of  As shown in the current-voltage characteristics (Fig. R13a), the contact resistance with Ag electrode is much larger than that of Au electrodes, which is caused by the high Schottky barrier at the Ag electrode/perovskite interface. In addition, we fabricated a device with asymmetric electrodes (Ag/perovskite/Au), whose current-voltage curve deviated from the origin, which also indicated the existence of the Schottky barrier.

Q4: A cross-section SEM will help the description of the configuration.
Response: Thank you for the valuable comment. Because the perovskite film thickness is only 500 nm and the channel length is 100 μm, it is difficult to show the specific details of the device in a low-magnification scanning electron microscopy (SEM) image (Fig. R14a). SEM image showing detailed device structure is provided in Fig. R15 in the next response. We supplemented a detailed 3D diagram (Fig. R14b) and added it to our revised manuscript (page 7, line 150) and supplementary materials (Fig. S4) to describe the feature size of the device, where the channel length is 100 μm, the channel width is 1000 μm, and the thickness of the perovskite film is 500 nm.  Response: We appreciate for the thoughtful comments and meaningful suggestions. We have carefully read your comments and noticed your concerns. According to your suggestion, we have made corresponding changes in the manuscript. We hope our reply can address your concerns.

Q1: In a recent paper by the author, a similar control method was used, that is, the introduction of additional control electrodes to suppress the dark current of the X-ray detector. Compared with this manuscript, what are the innovations in physical mechanism and technical methods?
Response: Thank you very much for your careful review. In our recent published work, the perovskite/C60/In2O3 heterojunction structure was applied. In this manuscript, we developed a simpler device structure with Au electrodes directly contacting with perovskite, and provided a more general physical model to reduce dark current, rendering it more universal and having a wider application prospect. We verified the physical mechanism of the carrier transport process and dark current reduction using electrical field simulation, making this work more convincing and instructive. Furthermore, the ion migration suppression effect of control voltage is first proposed and demonstrated in this work. In terms of the technical methods, we fabricated a flexible active-matrix photodetector array and successfully demonstrated its application in wearable health monitoring and flexible imaging sensors.

Q2: The author introduces a third electrode on the other side of the photoconductor to suppress the dark current. The introduced third electrode is similar to the gate in the transistor, and
What is the difference between both them?
Response: Thanks for your critical comment. Compared with the field-effect transistor (FET) structure, our control electrode is directly in contact with the semiconductor material, while the gate electrode in a FET is separated from the semiconductor material by a dielectric layer (Fig. R16). Although the current between the source and drain electrodes can be modulated by the gate voltage in the transistor structure, its current value is always positive because the offstate current of the transistor is directly related to the channel resistance. The control electrode in our structure can directly participate in the current transport process between the source and drain electrodes, so it has a stronger control effect on the dark current, and the value of the dark current can be reduced to zero or even a negative value (Fig. 2c). We appreciate for the constructive advice and have added the comments in our revised manuscript (page 3, line 70) and supplementary materials (Figs. S1).

Q3:
The author mentioned that the iodine ion migration was inhibited by introducing the control electrode, but no direct evidence was given to prove this.
Response: Thanks to the reviewer for pointing out this important point. To demonstrate the effective suppression of ion migration by our method, we characterized the perovskite films by in situ photoluminescence (PL) test, since the intensity change of PL can directly prove the occurrence of ion migration [Nat. Mater. 14, 193-198 (2015)] [Nat. Commun. 9, 5113 (2018)].
In addition, the edge of the signal electrode is the region where ion migration is most likely to occur because of its highest electric field strength [Energy Environ. Sci. 15, 5324-5339 (2022)]. 20 Fig. R17a and b show the schematic diagram of measuring the PL intensity of perovskite thin films around signal electrodes using the confocal microscope spectrometer (Alpha300, WITec).
After 0.5 V bias for 60 min, the PL intensity of conventional photoconductive-type FPD decreased significantly, while the PL intensity of the electrical field modulated FPD was almost unchanged, which proves that our method has a significant inhibitory effect on ion migration (Fig. R17c,d). The corresponding results and discussions have been incorporated into the revised manuscript (page 6, line 132) and supplementary materials ( Figure S3).   In order to explore the effect of control voltage (0.1 V) on the mobility of transistors, we measured the transfer characteristic curves of transistors under different conditions. As shown in Fig. R19b, the transfer curve of the transistor is almost independent of the control voltage because the capacitance between the control electrode and the In2O3 semiconductor is smaller than that between the gate electrode and the In2O3 semiconductor. The capacitance can be calculated by: = where is the permittivity of free space, is the relative permittivity, is the area, and is the thickness. The thickness of SiO2 is 100 nm, and the thickness of SU-8 photoresist is 5 μm (Fig. R19c) Fig. 3h and Fig. 5f, the reason why the photocurrent rises first and then falls?
Response: Thank you for your careful review and valuable comment. We have noticed the phenomenon that the photocurrent rises first and then falls. This phenomenon could be attributed to the unstable electrical pulse signals generated by the function generator, which is utilized to drive the light source (semiconductor laser).
To confirm this, we did comparison experiments by using a mechanical chopper (SSIinstrument, OE3001) as a modulation method to provide optical pulse inputs and measured the response of the photoconductor, under the same light power density condition (R20a, ii). In this test setup, the photocurrent becomes stable (Fig. R20b).
This proves that the current noise could be attributed to the input electrical pulse signals, rather than the device. Its corresponding explanation and discussion have been added to the revised manuscript (page 11, line 260) and supplementary materials (Fig. S17).

Q7:
The author shows a response pulse of photoconductivity under extremely weak light intensity in the supporting information Fig. 10. It is suggested that the author increase at least 10 stable response periods to increase credibility.

Response:
We thank the referee for this helpful suggestion. The corresponding weak light response data for twenty cycles of the FPD (Fig. R21) has been added to the supplementary material (Fig. S13), and its obvious photocurrent response demonstrates its excellent stability. nm and antisolvent CB added at 5 seconds countdown. However, when the antisolvent CB was added in advance (at 25 s countdown), the film roughness increased significantly (52.9 nm), as shown in Fig. R22b.
To further evaluate the influence of the surface roughness of the perovskite film on the voltage selection of the control electrode, we measured the dark current as a function of the control voltage (working voltage was fixed at 0.1 V). Fig. R22c shows that the dark current is suppressed to near zero when the control voltage is 90 mV (critical voltage<working voltage), because the sharp part of the surface of the rough perovskite film will lead to a higher local electric field than the flat surface, resulting in a stronger control effect [Appl. Phys. Lett. 89,25 133506 (2006)] [Phys. Rev. B 60, 9157-9164 (1999)]. Therefore, when the control voltage is set to 0.1 V, the current-time curve of the dark current stabilizes at a negative value (-28 pA), as shown in Fig. R22d.
These results illustrate that the film quality (e.g., surface roughness) could influence the selection of the control voltage, and smaller voltage values could be utilized when film has larger roughness values due to the enhanced local electric field (at sharp region). Relevant comments and results have been added to the revised manuscript (page 8, line 178) and supplementary material ( Figure S11).